Class D amplifiers use a low pass filter before the load (loudspeaker) to convert the amplified digital signal into an audio signal. The load may include speakers with impedance ranging from 2-16 .OMEGA.. A typical low pass filter of a class D amplifier is strongly dependent upon load impedance. An example of the typical low pass filter is shown in FIGS. 1 and 4. It includes the inductor L and the capacitor C.sub.LP. The low pass filter has a Laplace transfer function as follows: ##EQU1##
In conventional Butterworth, Bessel and linear phase filters, the terms s.sup.2 LC and Ls/R are on the same order of magnitude. Thus, the transfer function depends strongly on the load impedance. The first step for creating a transfer function that is independent of load impedance is to make sure that the term s.sup.2 LC&gt;&gt;Ls/R. Unfortunately, this approach results in a high quality (Q) factor filter which exhibits peaking at a resonant frequency ##EQU2##
as shown in FIG. 2. Such peaking is unacceptable in audio amplifiers where the gain must be flat throughout the audio bandwidth.
One prior art solution to the problem places an impedance balancing filter, also known as a Zobel filter, at the output. An example of a Zobel filter is shown in FIG. 3A. Its frequency response is shown in FIG. 3B. Although the Zobel filter reduces the Q of the low pass output filter and results in less peaking, it is costly and inefficient because the resistor in the Zobel network dissipates a significant fraction of the carrier as heat. This dissipation increases the cost of the Zobel network because the components must be chosen to handle this power. Since the Zobel network has a low impedance at ultrasonic frequencies, it limits the power bandwidth of the class D amplifier. If the input signal is not band limited, the Zobel may overheat and fail. The amplifier may still function, but without the Zobel, the peaking at high frequency will be audible.